stormwater mgJ00300R1V1_25_Kauri_St_Cooroy_Complete · MCU18/0165

25 Kauri St Cooroy




4.3      Validation
The un-mitigated developed case peak flows from the site have been validated against a QUDM
Rational Method calculation. This Rational Method calculation is provided in Appendix C. Table
4-5 compares the WBNM model peak flows with those of the Rational Method. This is
considered a suitable validation of the WBNM peak flows.

Table 4-5      Peak Flow Comparison - Base Case Vs Un-Mitigated Developed Case

                                           WBNM
                                        Un-Mitigated       Difference            Difference
   Event             Rational Method
                                       Developed Case           3
                                                             (m /s)                 (%)
                                             3
                                           (m /s)
  1% AEP                  0.30              0.30                0                   0%
  10% AEP                 0.17              0.19              0.02                  12%
  50% AEP                 0.09              0.11              0.02                  22%



4.4      Mitigated Developed Case
Modelling of the un-mitigated developed case described in the previous section demonstrates
that a detention basin is required to mitigate the increased peak runoff from the site.

It is proposed to incorporate an on-site detention storage that will receive runoff from the site
stormwater network. It has been assumed that flows in excess of the 5% AEP event (Q20) will
bypass the storage basin and discharge from the site as overland flow. This is necessary
because the proposed site roof and stormwater network will be designed to capture events up to
the 5% AEP only (design to be carried out at future detailed design phase). A bypass system
has been incorporated into the WBNM model to achieve this. The peak 5% AEP discharge from
the un-mitigated developed site is predicted by the WBNM model to be 0.24m3/s. Site discharge
is excess of this value will bypass the WBNM basin.

The sub-catchment areas and fraction impervious values are as per those of the Un-Mitigated
developed case.

The onsite detention storage and outlet details have been iteratively sized to achieve a non-
worsening of peak runoff. Details of the adopted configuration is provided in Table 4-6.




J00300R1V1                                                                                    8